1. Field of the Invention
This invention relates to a displacement information detecting apparatus, and particularly to the field of displacement measurement for optically measuring displacement information such as the amount of displacement of an object and the displacement speed of an object.
2. Related Background Art
An encoder has heretofore been widely used to measure the amount of displacement, the displacement speed, etc. of an object. Most of methods of optically measuring them use a movable grating and a reference grating.
(a-1) Generally, the reference grating is provided integrally with a light source and a light receiving portion, and is also called a fixed grating.
(a-2) On the other hand, as a special detecting system, there is proposed a displacement measuring apparatus having a detecting optical system by a construction using a portion of a movable grating as a reference grating as disclosed in Japanese Patent Publication No. 36-11793, i.e., a so-called self-projection type construction.
Particularly, an encoder is of a simple construction and yet can detect an amount of angular displacement, an amount of movement, etc. and is therefore widely used in machine tools and measuring apparatuses.
FIG. 1 of the accompanying drawings is a schematic view of the essential portions of an encoder proposed in Japanese Patent Publication No. 36-11793. In FIG. 1, a light beam from a light source 201 passes through an opening 202a formed in a base plate 202, whereafter it enters a first area 204a of an optical scale 204 as a displaceable object provided with a radial grating (linear grating) as a substantially parallel light beam by means of a lens system 203.
The light from the grating of the first area 204a of the optical scale 204 is introduced to a second area 204b of the optical scale 204 through a lens system 205, a corner cube 206 and a lens system 207, and forms a grating image on the surface thereof. The light beam which had passed through the second area 204b is condensed by a lens system 208 and is detected by a light receiving element 209.
When at this time, the optical scale 204 moves in the direction of arrow 204c, the image formed on the optical scale 204 is displaced in the opposite direction and therefore, a signal of a period twice as great as the grating resolving power (grating pitch) is obtained in the light receiving element 209.
The encoder shown in FIG. 1 obtains the displacement information of the optical scale 204 by the utilization of the periodic signal obtained by the light receiving element 209.
A system using this self-projection type detecting optical system has the following features:
(b-1) A detecting head which does not depend on the grating pitch can be realized.
(b-2) A periodic signal of double multiplication of the grating resolving power is obtained.
Industrially, the feature (b-1) has a great merit in the market of encoder of which various resolving powers are required, and a detecting head of high flexibility having no resolution dependency can be provided, and the lower cost by mass production is possible. Also, high resolving power detection is possible by the feature (b-2).
Because of a position detecting system having the detecting optical system by the self-projection type construction having many excellent features as described above, there is required a method of obtaining a phase difference signal of 2CH or greater necessary as an encoder by a more compact construction.
It is a first object of the present invention to provide, in a displacement measuring apparatus having the above-described self-projection type detecting optical system according to the conventional art, an information detecting apparatus using a phase difference detecting system which can be made more compact.
It is a second object of the present invention to provide a displacement information detecting apparatus suitable for a linear encoder or the like which can reduce the space for the disposition of each member and can achieve the simplification of the entire apparatus and can alleviate the allowable error of the disposition of each member and yet can detect the displacement information of a displaceable object.
Other objects of the present invention will become apparent from the following detailed description of some embodiments of the invention.